Scientists have created the first map that shows how the Greenland Ice Sheet has moved over time, revealing that ice in the interior is moving more slowly toward the edges than it has, on average, during the past 9,000 years. The findings, which researchers said don’t change the fact that the ice sheet is losing mass overall and contributing to sea level rise, are published in the Feb. 5 issue of Science. Along Greenland’s periphery, many glaciers are rapidly thinning. However, the vast interior of Greenland is slowly thickening, a process the new study clarifies.

“Scientists are very interested in understanding how ice sheets flow and how that flow may have been different in the past. Our paleo-velocity map for Greenland allows us to assess the flow of the ice sheet right now in the context of the last several thousand years,” said lead author Joe MacGregor of The University of Texas at Austin’s Institute for Geophysics (UTIG), a research unit of the Jackson School of Geosciences. The study builds on earlier UTIG-led research that developed a database of the many layers within Greenland’s ice sheet. Using this database, the scientists determined the flow pattern for the past 9,000 years — in effect creating a “paleo-velocity” map.

The authors identified three causes for this deceleration. First is that snowfall rates were generally higher during the past 9,000 years, second is the slow stiffening of the ice sheet over time, and third is the collapse of an “ice bridge” that used to connect Greenland’s ice to that on nearby Ellesmere Island. Of most interest were the last two. “Like many others, I had in mind the ongoing dramatic retreat and speedup along the edges of the ice sheet, so I’d assumed that the interior was faster now too. But it wasn’t,” said MacGregor.In comparing the paleo-velocity map with modern flow rates, researchers found that the ice sheet’s interior is moving more slowly now than during most of the Holocene, a geological period that began about 11,700 years ago and runs to the present.

“The ice that formed from snow that fell in Greenland during the last ice age is about three times softer than the ice being formed today,” according to William Colgan of York University’s Lassonde School of Engineering, a co-author of the study. Because of this difference, the ice sheet is slowly becoming stiffer. As a consequence, the ice sheet is flowing more slowly and getting thicker over time. This effect is most important in southern Greenland, where higher snowfall rates have led to rapid replacement of ice from the last glacial period with more modern Holocene ice. “But that didn’t explain what was happening elsewhere in Greenland, particularly the northwest, where there isn’t as much snowfall, so the stiffening effect isn’t as important,” said MacGregor.

The explanation of deceleration in the northwest lies in the collapse 10,000 years ago of an “ice bridge” across Nares Strait, which used to connect Greenland’s ice to that on Ellesmere Island. The collapse of the ice bridge at the end of the last ice age led to acceleration in the northwest, but the ice sheet has since returned to a slower pace.

These changes, which started thousands of years ago, affect our understanding of the changing Greenland Ice Sheet even today. Scientists often use GPS and altimeters aboard satellites to measure the elevation of the ice surface and study how much mass is being lost or gained across the ice sheet. When correcting for other known effects on the surface elevation, any leftover thickening is assumed to be due to increasing snowfall, but this study shows that may not be the case. “We’re saying that recent increases in snowfall do not necessarily explain present-day interior thickening,” said Colgan. “If you’re using a satellite altimeter to figure out how much mass Greenland is losing, you’re going to get the answer slightly wrong unless you account for these very long-term signals that are evident in its interior.”

Scientists studying data from the top of the Greenland ice sheet have discovered that during winter in the center of the world’s largest island, temperature inversions and other low-level atmospheric phenomena effectively isolate the ice surface from the atmosphere — recycling water vapor and halting the loss or gain of ice. A team of climate scientists made the surprising discovery from three years of data collected at Summit Camp, an arid, glaciated landscape 10,500 feet above sea level in the middle of the Greenland ice sheet. “This is a place, unlike the rest of the ice sheet, where ice is accumulating,” says Max Berkelhammer, assistant professor of earth and environmental sciences at the University of Illinois at Chicago. Berkelhammer is first author on the study, reported in Science Advances, an open-access online publication of the journal Science. Near Greenland’s coasts, Berkelhammer said, “it’s relatively warm, and the ice melts faster and faster.”

“But in the center of the ice sheet, it’s 25 below zero Celsius (-13 F), so it’s always freezing, even if it warms. It’s a very rare occurrence to go above freezing,” he said. The authors note that “despite rapid melting in the coastal regions of the ice sheet, a significant area — approximately 40 percent — rarely experiences surface melting.” Solid ice can be lost not only by melting into liquid water. Under certain conditions, it can vaporize by sublimation, a one-step transition from solid to gas. Such conditions exist at the high-altitude, dry, frigid surface of Greenland’s interior. “Sublimation is common there, unlike other places,” Berkelhammer said. “We looked at the exchange of water between the ice sheet and the air above it through condensation, evaporation, and sublimation.”

At Summit Camp, a 150-foot tower was used to draw air samples at various heights above the surface and pipe the air into a laboratory buried a few feet below the ice. Lasers analyzed the air for two different isotopes of oxygen in H2O, whose ratio indicates the temperature at which the water molecules became airborne. “We noticed a specific process that was occurring, where low-level fog would form right above the surface of the ice sheet,” Berkelhammer said. A fogbow – a rainbow caused by fog – often appeared. “As ice sublimates from the surface, it forms a fog,” he said. “As the particles get heavier and settle back to the surface, you get recycling, rather than dissipation that would remove ice.” In winter, 80 percent of the ice that would otherwise be lost is recycled, Berkelhammer said. “So it’s an incredibly efficient process.”

But many questions remain as to how this boundary-layer recycling contributes to models of climate change. We expected sublimation to increase with temperature, but we find no net loss” of ice over time, Berkelhammer said, again referring just to the interior of the ice mass. “You could say, if this process changes, you’d lose ice significantly faster. Or, if (recycling) becomes even more efficient, you would conserve even more ice mass. “We can’t predict,” he said. “And we don’t know from the ice-core records what the history is.” The next step, he said, is to run experiments to see how sublimation changes with temperature associated with past and future changes in atmospheric carbon dioxide levels, to see how recycling fits into climate models.

“If we want to model how the ice sheet is warming, we need to include everything we know,” he said. “This is a new process to incorporate in models.” But Berkelhammer cautions against over-interpreting the recycling as good news for the ice sheet or the planet, as its overall effect is likely to be relatively minor. “This is small potatoes compared to the calving that’s going on along the coasts,” he said. “Every time we go back to Greenland, the edge of the ice is farther away from the coast.”

Top: The total daily contribution to the surface mass balance from the entire ice sheet (blue line, Gt/day). Bottom: The accumulated surface mass balance from September 1st to now (blue line, Gt) and the season 2011-12 (red) which had very high summer melt in Greenland. For comparison, the mean curve from the period 1990-2013 is shown (dark grey). The same calendar day in each of the 24 years (in the period 1990-2013) will have its own value. These differences from year to year are illustrated by the light grey band. For each calendar day, however, the lowest and highest values of the 24 years have been left out.

Brief Communication: Glaciers in the Hunza Catchment (Karakoram) are in balance since the 1970sPrevious geodetic estimates of mass changes in the Karakoram revealed balanced budgets or a possible slight mass gain since the year ~ 2000. Indications for longer-term stability exist but no mass budget analyses are available before 2000. Here, we show that glaciers in the Hunza River basin (Central Karakoram) were on average in balance since the 1970s based on analysis of stereo Hexagon KH-9, SRTM, ASTER and Cartosat-1 data. Heterogeneous behaviour and frequent surge activities were also characteristic for the period before 2000.

Ages of major Little Ice Age glacier fluctuations on the southeast Tibetan Plateau derived from tree-ring-based moraine datingAdvances and retreats of glaciers on the Tibetan Plateau (TP) are widely regarded as indicators for climate changes sensitive to macroclimatic forcing mechanisms like the Asian summer monsoon. However, it often remains unclear why some glaciers retreat or advance earlier than others, particularly regarding the timing of the so-called ‘Little Ice Age (LIA) maximum’. Within this study, we present newly acquired tree-ring data from four glacier forefields in the Nyainqêntanglha Range on the southeastern TP and link them to previous studies. Two of the glaciers formed a double-tongue moraine set during the LIA, with trees providing minimum ages for two different major glacier extents. In combination with evidence from a third glacier, three warm phases within the generally cold LIA can be deduced during the first half of the 16th century, and the mid-17th and 18th centuries. This implies the occurrence of three preceding major cold spells. A minor cool phase between 1790 and 1850 marks the end of the LIA on the southeast TP. Minimum ages for beginning retreat from the last major LIA glacial advance differ by only 40 years for all sampled monsoonal temperate valley glaciers in the Nyainqêntanglha Range. This implies a common forcing, whose impact on glacier dynamics is only slightly altered by topographical factors or local climatic conditions. Regression analysis suggests that the delay of the glacier retreat from the maximum extent is mainly related to glacier size, with minor influence of ablation area aspect. All moraine ages highlight intermediate LIA warm phases on the southeast TP corresponding to warm phases on the northern Hemisphere and thus prove monsoonal temperate glaciers to be highly sensitive archives for northern hemispheric climatic conditions.

“ These results show that the linear relationship between Arctic sea-ice loss and mid-latitude weather patterns is weak, suggesting that the remote atmospheric response is small compared with the internal variability, or highly nonlinear with respect to the sea-ice area anomalies.”

“Thus, our results do not show evidence of an unusually elongated jet stream associated with Arctic sea-ice loss on a monthly time scale.”

“We have shown using several different metrics that the remote atmospheric response can be non-robust due to internal dynamics alone, and leave diagnosis of mechanisms behind this non-robustness for future studies. “

The robustness of mid-latitude weather pattern changes due to Arctic sea-ice lossThe significance and robustness of the link between Arctic sea-ice loss and changes in mid-latitude weather patterns is investigated through a series of model simulations in Community Atmosphere Model 5.3 with systematically perturbed sea-ice cover in the Arctic. Using a large ensemble of ten sea-ice scenarios and 550 simulations, it is found that prescribed Arctic sea-ice anomalies produce statistically significant changes for certain metrics of the mid-latitude circulation but not for others. Furthermore, the significant mid-latitude circulation changes do not scale linearly with the sea-ice anomalies, and are not present in all scenarios, indicating that the remote atmospheric response to reduced Arctic sea ice can be statistically significant under certain conditions, but is generally non-robust. Shifts in the Northern Hemisphere polar jet stream and changes in the meridional extent of upper-level large-scale waves due to the sea ice perturbations are generally small and not clearly distinguished from intrinsic variability. Reduced Arctic sea ice may favor a circulation pattern that resembles the negative phase of the Arctic Oscillation, and may increase the risk of cold outbreaks in eastern Asia by almost 50 %, but this response is found in only half of the scenarios with negative sea-ice anomalies. In eastern North America the frequency of extreme cold events decreases almost linearly with decreasing sea-ice cover. Our finding of frequent significant anomalies without a robust linear response suggests interactions between variability and persistence in the coupled system, which may contribute to the lack of convergence among studies of Arctic influences on mid-latitude circulation.

Calendar-dated glacier variations in the western European Alps during the Neoglacial: the Mer de Glace record, Mont Blanc massifHolocene glacier records from the western European Alps are still sparse, although a number of sites are well suited to constraining pre- and early- Little Ice Age (LIA) glacier advances. The present study provides the first dendrochronologically-based and calendar-dated Neoglacial glacier chronology for the Mont Blanc massif, French Alps. It is based on the analysis of over 240 glacially buried Pinus cembra subfossil logs and wood remains found either embedded-in-till or as detrital material in the Mer de Glace right lateral moraine. Only a few of the samples were found to be ‘formally in situ’ but we show that some logs were ‘virtually in situ’ (not rooted but showing little or no evidence of reworking) and could be used to accurately reconstruct past glacier margin behavior in space and time. Uncertainties regarding the other samples may relate to original growth location and/or to outer wood decay. The resulting dates (followed by a ‘+’) were therefore considered maximum-limiting ages for glacier advances. The main burial events – interpreted as glacier advances – occurred between ca 1655+ and 1544+ BC, between ca 1230+ and 1105+ BC, between ca 1013+ and 962+/937+ BC, at ca 802–777 BC, after 608+ BC, between 312 and 337 AD, between ca 485+ AD and 606+ AD, between 1120 and 1178 AD, between ca 1248 and 1278+/1296 AD, and after 1352+ AD. These advances predate the late LIA maxima known from historical sources. The magnitude of the advances gradually increased to culminate in three near-Neoglacial maxima during the 7th, 12th and 13th centuries AD, followed by a first LIA/Neoglacial maximum in the second half of the 14th century AD. The pattern of Neoglacial events described here is coherent with Central and Eastern Alpine glacier chronologies. This indicates marked synchronicity of late Holocene glacier variability and forcing at a regional scale, although occasional differences could be detected between ‘Western’ and ‘Eastern’ records. The Mer de Glace record also confirms the link between the timing of sediment erosion in a high-elevation glaciated Alpine catchment and subsequent deposition in the sub-alpine Lake Bourget.

Cold spell gripped Europe 3,000 years before ‘Little Ice Age,’ says studyHuman civilization arose during the relatively balmy climate of the last 10,000 years. Even so, evidence is accumulating that at least two cold spells gripped the northern hemisphere during this time, and that the cooling may have coincided with drought in the tropics. Emerging research on climate during this Holocene period suggests that temperature swings were more common than previously thought, and that climate changes happened on a broad, hemispheric scale.

In a new study in Geology, Irene Schimmelpfennig, a postdoctoral researcher at Columbia University’s Lamont-Doherty Earth Observatory, and her colleagues find that one glacier in the Western Alps was bigger than today during at least two periods in the last 10,000 years—during the well-documented Little Ice Age between 1300 AD and 1850 AD, and at an earlier time, from 3,800 to 3,200 years ago.

Between 2006 and 2010, Schimmelpfennig and colleagues traveled to Switzerland’s Tsidjiore Nouve Glacier to collect rocks dragged downhill by past advancing glaciers. As temperatures warmed and the ice retreated, the ridges of rock or sediment, or moraines, left behind were bombarded by cosmic rays.

Greenland’s shrunken ice sheet: We’ve been here beforeClues in the Arctic fossil record suggest that 3-5,000 years ago, the ice sheet was the smallest it has been in the past 10,000 years

Think Greenland’s ice sheet is small today? It was smaller — as small as it has ever been in recent history — from 3-5,000 years ago, according to scientists who studied the ice sheet’s history using a new technique they developed for interpreting the Arctic fossil record. “What’s really interesting about this is that on land, the atmosphere was warmest between 9,000 and 5,000 years ago, maybe as late as 4,000 years ago.The oceans, on the other hand, were warmest between 5-3,000 years ago,” said Jason Briner, PhD, University at Buffalo associate professor of geology, who led the study. “What it tells us is that the ice sheets might really respond to ocean temperatures,” he said. “It’s a clue to what might happen in the future as the Earth continues to warm.”

The findings appeared online on Nov. 22 in the journal Geology. Briner’s team included Darrell Kaufman, an organic geochemist from Northern Arizona University; Ole Bennike, a clam taxonomist from the Geological Survey of Denmark and Greenland; and Matthew Kosnik, a statistician from Australia’s Macquarie University. The study is important not only for illuminating the history of Greenland’s ice sheet, but for providing geologists with an important new tool: A method of using Arctic fossils to deduce when glaciers were smaller than they are today.

Scientists have many techniques for figuring out when ice sheets were larger, but few for the opposite scenario. “Traditional approaches have a difficult time identifying when ice sheets were smaller,” Briner said. “The outcome of our work is that we now have a tool that allows us to see how the ice sheet responded to past times that were as warm or warmer than present — times analogous to today and the near future.” The technique the scientists developed involves dating fossils in piles of debris found at the edge of glaciers. To elaborate: Growing ice sheets are like bulldozers, pushing rocks, boulders and other detritus into heaps of rubble called moraines. Because glaciers only do this plowing when they’re getting bigger, logic dictates that rocks or fossils found in a moraine must have been scooped up at a time when the associated glacier was older and smaller. So if a moraine contains fossils from 3,000 years ago, that means the glacier was growing — and smaller than it is today — 3,000 years ago.

This is exactly what the scientists saw in Greenland: They looked at 250 ancient clams from moraines in three western regions, and discovered that most of the fossils were between 3-5,000 years old. The finding suggests that this was the period when the ice sheet’s western extent was at its smallest in recent history, Briner said. “Because we see the most shells dating to the 5-3000-year period, we think that this is when the most land was ice-free, when large layers of mud and fossils were allowed to accumulate before the glacier came and bulldozed them up,” he said.

Because radiocarbon dating is expensive, Briner and his colleagues found another way to trace the age of their fossils. Their solution was to look at the structure of amino acids — the building blocks of proteins — in the fossils of ancient clams. Amino acids come in two orientations that are mirror images of each other, known as D and L, and living organisms generally keep their amino acids in an L configuration. When organisms die, however, the amino acids begin to flip. In dead clams, for example, D forms of aspartic acid start turning to L’s. Because this shift takes place slowly over time, the ratio of D’s to L’s in a fossil is a giveaway of its age. Knowing this, Briner’s research team matched D and L ratios in 20 Arctic clamshells to their radiocarbon-dated ages to generate a scale showing which ratios corresponded with which ages. The researchers then looked at the D and L ratios of aspartic acid in the 250 Greenland clamshells to come up with the fossils’ ages. Amino acid dating is not new, but applying it to the study of glaciers could help scientists better understand the history of ice — and climate change — on Earth.

Earliest Holocene south Greenland ice sheet retreat within its late Holocene extentEarly Holocene summer warmth drove dramatic Greenland ice sheet (GIS) retreat. Subsequent insolation-driven cooling caused GIS margin readvance to late Holocene maxima, from which ice margins are now retreating. We use 10Be surface exposure ages from four locations between 69.4°N and 61.2°N to date when in the early Holocene south to west GIS margins retreated to within these late Holocene maximum extents. We find that this occurred at 11.1 ± 0.2 ka to 10.6 ± 0.5 ka in south Greenland, significantly earlier than previous estimates, and 6.8 ± 0.1 ka to 7.9 ± 0.1 ka in southwest to west Greenland, consistent with existing 10Be ages. At least in south Greenland, these 10Be ages likely provide a minimum constraint for when on a multicentury timescale summer temperatures after the last deglaciation warmed above late Holocene temperatures in the early Holocene. Current south Greenland ice margin retreat suggests that south Greenland may have now warmed to or above earliest Holocene summer temperatures.

Slow retreat of a land based sector of the West Greenland Ice Sheet during the Holocene Thermal Maximum: evidence from threshold lakes at PaakitsoqRecords from two connected proglacial threshold lakes at Paakitsoq, west Greenland have been analyzed in order to investigate the response of a land-based ice sheet margin to Holocene climate change. The results are used to test whether or not the land terminating margin at Paakitsoq behaved synchronously with the nearby marine terminating Jakobshavn Isbræ during the Holocene. The radiocarbon dated lake sediment cores indicate that the ice margin retreated to its present position ∼6.5 ka ago and thereafter maintained a relatively stable configuration similar to the present for >1000 years, despite summer temperatures >2 °C higher than today. The lakes became non-glacial after 5.4 cal. ka BP, when the ice margin retreated behind a drainage divide situated ∼1.5 km inland of the present margin. By this time, Jakobshavn Isbræ had already reached its minimum configuration. The Paakitsoq ice margin remained >1.5 km inland of its present margin until after 240 ± 20 cal. yr BP; after this time, both Paakitsoq ice margin and Jakobshavn Isbræ reached their Holocene maxima during the later stage of the Little Ice Age. Our results suggest that the present ice margin position at Paakitsoq is relatively stable in a warming climate but after a total retreat of ∼1.5 km behind the present margin position it may become marine based and more unstable due to marine melting and calving processes.

Each summer, Greenland’s ice sheet — the world’s second-largest expanse of ice, measuring three times the size of Texas — begins to melt. Pockets of melting ice form hundreds of large, ‘supraglacial’ lakes on the surface of the ice. Many of these lakes drain through cracks and crevasses in the ice sheet, creating a liquid layer over which massive chunks of ice can slide. This natural conveyor belt can speed ice toward the coast, where it eventually falls off into the sea. Now researchers have found that while warming temperatures are creating more inland lakes, these lakes cannot drain their water locally, as lakes along the coast do, and are not likely to change the amount of water reaching the ground in inland regions.

Each summer, Greenland’s ice sheet — the world’s second-largest expanse of ice, measuring three times the size of Texas — begins to melt. Pockets of melting ice form hundreds of large, ‘supraglacial’ lakes on the surface of the ice. Many of these lakes drain through cracks and crevasses in the ice sheet, creating a liquid layer over which massive chunks of ice can slide. This natural conveyor belt can speed ice toward the coast, where it eventually falls off into the sea.

In recent years, scientists have observed more lakes forming toward the center of the ice sheet — a region that had been previously too cold to melt enough ice for lakes to form. The expanding range of lakes has led scientists to wonder whether Greenland will ultimately raise global sea levels higher than previously predicted.

Now researchers at MIT, Woods Hole Oceanographic Institution (WHOI), and elsewhere have found that while warming temperatures are creating more inland lakes, these lakes cannot drain their water locally, as lakes along the coast do, and are not likely to change the amount of water reaching the ground in inland regions.

‘It’s essentially a check on the inner ice starting to move along this fast conveyor belt,’ says Laura Stevens, a graduate student in MIT’s Department of Earth, Atmospheric and Planetary Sciences. ‘One of the big questions about the Greenland ice sheet is how much of the ice sheet [travels towards the coast] during the summer, and how much is entering into the ocean. Our hypothesis that inland lakes are less likely to drain locally suggests the ice sheet in that region won’t speed up. That’s good news, at least for the time being.’

Stevens and her colleagues, including Thomas Herring, a professor of geophysics at MIT, have published their results today in the journal Nature.

A trickle and a trigger

In summer 2006, Sarah Das, a glaciologist at WHOI, led a team to document the drainage of North Lake, a 10-meter-deep, 2-kilometer-wide lake on the western side of Greenland. The group observed that each summer, the lake, like many others, drained quickly, completely emptying in just a couple of hours.

‘You can hear the water rushing down in the distance, and even if you’re a couple kilometers away, you see all these microcracks running along the ground around you,’ Stevens says.

The researchers set up one GPS station near the lake to record the surface of the ice during its draining, and later identified a large fracture in the basin through which the water drained. However, it wasn’t clear what triggered the fracture that caused the lake to drain so quickly.

Das returned to Greenland in summer 2011, along with Stevens and others, to get a more detailed picture of the lake’s seasonal draining. The team set up 16 GPS stations in two rings around the lake, and recorded the movement of the ice as the lake drained once each summer over three consecutive summers.

From the GPS data, they observed a period of six to 12 hours, just before the lake drained, in which some water from the lake trickled to the bottom of the ice sheet through ‘moulins’ — narrow vertical channels in the ice. During this brief period, the researchers observed water collecting at the bottom of the ice sheet, pushing up on the surface ice. This initial pooling of water seemed to trigger the rest of the lake to drain.

‘That water will cause the ice above it to be jacked up like a dome, and then you’ve created tension at the surface that allows the ice sheet to start to fracture,’ Stevens says. ‘Once a fracture gets beneath the lake, then water just starts to pour into that fracture, and the whole thing goes.’

A check on runaway lake drainage

North Lake is located within the coastal region of Greenland, where the ice sheet is thinner, and more moulins route water at the surface of the ice sheet to its base. In contrast, lakes further inland are higher in elevation and form over thicker ice. Stevens says it’s unlikely that inland lakes would drain, as there are fewer moulins near inland lakes, which prevents water from getting to the ground locally. Without these trigger channels, larger fractures would not form in the lake basin, and lakes would stay intact, simply refreezing in the winter or overflowing into a surface stream.

‘It is critical to understand how and why these lakes drain in order to predict how much mass the ice sheet will contribute to sea-level rise in our warming climate,’ Stevens says. ‘We find that while lakes are forming inland, they probably won’t drain by this…mechanism. The inland lakes will more likely drain their water via surface stream runoff, which transfers the water to the bed in more coastal areas of the ice sheet. So, while we see inland ice beginning to speed up as more melt happens inland, the draining of inland lakes likely won’t exacerbate the situation.’

Persistent link between solar activity and Greenland climate during the Last Glacial MaximumChanges in solar activity have previously been proposed to cause decadal- to millennial-scale fluctuations in both the modern and Holocene climates1. Direct observational records of solar activity, such as sunspot numbers, exist for only the past few hundred years, so solar variability for earlier periods is typically reconstructed from measurements of cosmogenic radionuclides such as 10Be and 14C from ice cores and tree rings2, 3. Here we present a high-resolution 10Be record from the ice core collected from central Greenland by the Greenland Ice Core Project (GRIP). The record spans from 22,500 to 10,000 years ago, and is based on new and compiled data4, 5, 6. Using 14C records7, 8 to control for climate-related influences on 10Be deposition, we reconstruct centennial changes in solar activity. We find that during the Last Glacial Maximum, solar minima correlate with more negative δ18O values of ice and are accompanied by increased snow accumulation and sea-salt input over central Greenland. We suggest that solar minima could have induced changes in the stratosphere that favour the development of high-pressure blocking systems located to the south of Greenland, as has been found in observations and model simulations for recent climate9, 10. We conclude that the mechanism behind solar forcing of regional climate change may have been similar under both modern and Last Glacial Maximum climate conditions.

Ice sheets may be more resilient than thought, say Stanford scientists

Stanford study suggests that today’s ice sheets may be more resilient to increased carbon dioxide levels than previously thought.

By Miles Traer

Sea level rise poses one of the biggest threats to human systems in a globally warming world, potentially causing trillions of dollars’ worth of damages to flooded cities around the world. As surface temperatures rise, ice sheets are melting at record rates and sea levels are rising. But there may be some good news amid the worry. Sea levels may not rise as high as assumed. To predict sea level changes, scientists look to Earth’s distant past, when climate conditions were similar to today, and investigate how the planet’s ice sheets responded then to warmer temperatures brought on by increased carbon dioxide in the atmosphere.

In a recently published study in the journal Geology, PhD students Matthew Winnick and Jeremy Caves at Stanford School of Earth, Energy & Environmental Sciences explored these very old conditions and found that sea level might not have risen as much as previously thought – and thus may not rise as fast as predicted now. To better understand global sea level rise, Winnick and Caves analyzed the middle Pliocene warm period, the last time in Earth’s history, approximately 3 million years ago, when carbon dioxide levels in the atmosphere were close to their present values (350-450 parts per million). “The Pliocene is an important analogue for today’s planet not only because of the related greenhouse gas concentrations, but because the continents were roughly where they are today, meaning ocean and climate circulation patterns are comparable,” said Winnick. These similarities are why the Intergovernmental Panel on Climate Change (IPCC), the group responsible for global sea level rise projections, focuses on the mid-Pliocene warm period to inform their computer models.

Previous studies of the mid-Pliocene warm period used oxygen isotope records to determine the volume of Earth’s ice sheets and, by proxy, sea level. Effectively, the oxygen isotope records act as a fingerprint of Earth’s ice sheets.By combining the fingerprint with models of ice sheet meltwater, many previous researchers thought that sea level was likely 82 to 98 feet (25 to 30 meters) higher during the Pliocene. Such high sea level would require a full deglaciation of the Greenland Ice Sheet and the West Antarctic Ice Sheet, and as much as 30 percent of the East Antarctic Ice Sheet – enough to cover New York City under 50 feet of water. But these estimates arose because the researchers assumed that the Antarctic ice of the Pliocene had the same isotopic composition, that is, the same fingerprint, as it does today – an assumption that Winnick and Caves challenge in their new report.

To understand the isotopic composition of Pliocene ice, Winnick and Caves began in the present day using well-established relationships between temperature and the geochemical fingerprint. By combining this modern relationship with estimates of ancient Pliocene surface temperatures, they were able to better refine the fingerprint of the Antarctic ice millions of years ago. In re-thinking this critical assumption, and by extending their analysis to incorporate ice sheet models, Winnick and Caves recalculated the global sea level of the Pliocene and found that it was 30 to 44 feet (9 to 13.5 meters) higher, significantly lower than the previous estimate.

“Our results are tentatively good news,” Winnick said. “They suggest that global sea level is less sensitive to high atmospheric carbon dioxide concentrations than previously thought. In particular, we argue that this is due to the stability of the East Antarctic Ice Sheet, which might be more resilient than previous studies have suggested.” However, a rise in global sea level by up to 44 feet (13.5 meters) is still enough to inundate Miami, New Orleans and New York City, and threaten large portions of San Francisco, Winnick cautioned.

While the study helps refine our understanding of Pliocene sea level, both Winnick and Caves point out that it’s not straightforward to apply these results to today’s planet. “Ice sheets typically take centuries to millennia to respond to increased carbon dioxide, so it’s more difficult to say what will happen on shorter time scales, like the next few decades,” Winnick said. “Add that to the fact that CO2 levels were relatively consistent in the Pliocene, and we’re increasing them much more rapidly today, and it really highlights the importance of understanding how sea level responds to rising temperatures. Estimates of Pliocene sea level might provide a powerful tool for testing the ability of our ice sheet models to predict future changes in sea level.”

Greenland precipitation trends in a long-term instrumental climate context (1890–2012): evaluation of coastal and ice core recordsHere, we present an analysis of monthly, seasonal, and annual long-term precipitation time-series compiled from coastal meteorological stations in Greenland and Greenland Ice Sheet (GrIS) ice cores (including three new ice core records from ACT11D, Tunu2013, and Summit2010). The dataset covers the period from 1890 to 2012, a period of climate warming. For approximately the first decade of the new millennium (2001–2012) minimum and maximum mean annual precipitation conditions are found in Northeast Greenland (Tunu2013 c. 120 mm water equivalent (w.e.) year−1) and South Greenland (Ikerasassuaq: c. 2300 mm w.e. year−1), respectively. The coastal meteorological stations showed on average increasing trends for 1890–2012 (3.5 mm w.e. year−2) and 1961–2012 (1.3 mm w.e. year−2). Years with high coastal annual precipitation also had a: (1) significant high number of precipitation days (r2 = 0.59); and (2) high precipitation intensity measured as 24-h precipitation (r2 = 0.54). For the GrIS the precipitation estimated from ice cores increased on average by 0.1 mm w.e. year−2 (1890–2000), showing an antiphase variability in precipitation trends between the GrIS and the coastal regions. Around 1960 a major shift occurred in the precipitation pattern towards wetter precipitation conditions for coastal Greenland, while drier conditions became more prevalent on the GrIS. Differences in precipitation trends indicate a heterogeneous spatial distribution of precipitation in Greenland. An Empirical Orthogonal Function analysis reveals a spatiotemporal cycle of precipitation that is linked instantaneously to the North Atlantic Oscillation and the Atlantic Multidecadal Oscillation and with an ∼6 years lag time response to the Greenland Blocking Index.

Recent accumulation variability in northwest Greenland from ground-penetrating radar and shallow cores along the Greenland Inland TraverseAccumulation is a key parameter governing the mass balance of the Greenland ice sheet. Several studies have documented the spatial variability of accumulation over wide spatial scales, primarily using point data, remote sensing or modeling. Direct measurements of spatially extensive, detailed profiles of accumulation in Greenland, however, are rare. We used 400 MHz ground-penetrating radar along the 1009 km route of the Greenland Inland Traverse from Thule to Summit during April and May of 2011, to image continuous internal reflecting horizons. We dated these horizons using ice-core chemistry at each end of the traverse. Using density profiles measured along the traverse, we determined the depth to the horizons and the corresponding water-equivalent accumulation rates. The measured accumulation rates vary from ∼0.1 m w.e.a–1 in the interior to ∼0.7 m w.e.a–1 near the coast, and correspond broadly with existing published model results, though there are some excursions. Comparison of our recent accumulation rates with those collected along a similar route in the 1950s shows a 10% increase in accumulation rates over the past 52 years along most of the traverse route. This implies that the increased water vapor capacity of warmer air is increasing accumulation in the interior of Greenland.

Spatial and temporal distribution of mass loss from the Greenland Ice Sheet since AD 1900The response of the Greenland Ice Sheet (GIS) to changes in temperature during the twentieth century remains contentious1, largely owing to difficulties in estimating the spatial and temporal distribution of ice mass changes before 1992, when Greenland-wide observations first became available2. The only previous estimates of change during the twentieth century are based on empirical modelling3, 4, 5 and energy balance modelling6, 7. Consequently, no observation-based estimates of the contribution from the GIS to the global-mean sea level budget before 1990 are included in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change8. Here we calculate spatial ice mass loss around the entire GIS from 1900 to the present using aerial imagery from the 1980s. This allows accurate high-resolution mapping of geomorphic features related to the maximum extent of the GIS during the Little Ice Age9 at the end of the nineteenth century. We estimate the total ice mass loss and its spatial distribution for three periods: 1900–1983 (75.1 ± 29.4 gigatonnes per year), 1983–2003 (73.8 ± 40.5 gigatonnes per year), and 2003–2010 (186.4 ± 18.9 gigatonnes per year). Furthermore, using two surface mass balance models10, 11 we partition the mass balance into a term for surface mass balance (that is, total precipitation minus total sublimation minus runoff) and a dynamic term. We find that many areas currently undergoing change are identical to those that experienced considerable thinning throughout the twentieth century. We also reveal that the surface mass balance term shows a considerable decrease since 2003, whereas the dynamic term is constant over the past 110 years. Overall, our observation-based findings show that during the twentieth century the GIS contributed at least 25.0 ± 9.4 millimetres of global-mean sea level rise. Our result will help to close the twentieth-century sea level budget, which remains crucial for evaluating the reliability of models used to predict global sea level rise1, 8.

There is always time for rational skepticism: Reply to Hall et alIn this response to Hall et al., we addressed the various criticisms raised in their rejoinder about the validity of our arguments for climate-change skepticism. The so-called consensus on anthropogenic climate change is challenged, and doubts are casted regarding the credibility and accurateness of the IPCC reports. This response continues to demonstrate the serious drawbacks of current climate-change assessments, as well as to provide evidence undermining the alarmist assessments of climate-change impacts. Overall, the current state of knowledge supports our call for a skeptical approach in studies on climate change and tourism.

Climate scientists at the National Center for Atmospheric Research (NCAR) present evidence in a new study that they can predict whether the Arctic sea ice that forms in the winter will grow, shrink, or hold its own over the next several years.

The team of scientists has found that changes in the North Atlantic ocean circulation could allow overall winter sea ice extent to remain steady in the near future, with continued loss in some regions balanced by possible growth in others, including in the Barents Sea. “We know that over the long term, winter sea ice will continue to retreat,” said NCAR scientist Stephen Yeager, lead author of the new study, which appears in the journal Geophysical Research Letters. “But we are predicting that the rate will taper off for several years in the future before resuming. We are not implying some kind of recovery from the effects of human-caused global warming; it’s really just a slow down in winter sea ice loss.” The research was funded largely by the National Science Foundation, NCAR’s sponsor, with additional support from the National Oceanic and Atmospheric Administration and the U.S. Department of Energy. (weiterlesen …)